Co-extruded, multi-lumen medical lead

ABSTRACT

Medical electrical leads for sensing or electrical stimulation of body organs or tissues, particularly implantable cardiac leads for delivering pacing pulses and cardioversion/defibrillation shocks, and/or sensing the cardiac electrogram (EGM) or other physiologic data and their methods of fabrication are disclosed. A lead body sheath is co-extruded in a co-extrusion process using biocompatible, electrically insulating, materials of differing durometers in differing axial sections thereof, resulting in a unitary lead body sheath having differing stiffness sections including axial segments or webs or lumen encircling rings or other structures in its cross-section. The lead body sheath is co-extruded to have an outer surface adapted to be exposed to the environment or to be enclosed within an outer sheath and to have a plurality of lead conductor lumens for receiving and enclosing a like plurality of lead conductors of the same or differing types. The lead body sheath can be co-extruded of a plurality of sheath segments containing a lead conductor lumen and formed of a first durometer material or of differing durometer materials. A web of a further durometer material can be co-extruded extending between the adjoining boundaries of the axial sheath segments and bonding the adjacent segments together. The lead body sheath can be tailored to exhibit differing bending stiffnesses away from the lead body sheath axis in selected polar directions around the 360° circumference of the sheath body.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] Reference is hereby made to commonly assigned, U.S. patentapplication Ser. No. 08/990,647 filed Dec. 15, 1997, for MEDICALELECTRICAL LEAD in the name of Alan Rausch et al.

FIELD OF THE INVENTION

[0002] The present invention relates to medical electrical leads forsensing or electrical stimulation of body organs or tissues and theirmethod of fabrication, such leads having multiple electrical conductorsencased in a lead body, and particularly to implantable cardiac leadsfor delivering electrical stimulation to the heart, e.g., pacing pulsesand cardioversion/defibrillation shocks, and/or sensing the cardiacelectrogram (EGM) or other physiologic data.

BACKGROUND OF THE INVENTION

[0003] Implantable medical electrical stimulation and/or sensing leadsare well known in the fields of cardiac stimulation and monitoring,including cardiac pacing and cardioversion/defibrillation, and in otherfields of electrical stimulation or monitoring of electrical signals orother physiologic parameters of the body. A pacemaker orcardioverter/defibrillator implantable pulse generator (IPG) or acardiac monitor is typically coupled to the heart through one or more ofsuch endocardial leads. The proximal end of such leads typically isformed with a connector which connects to a terminal of the IPG ormonitor. The lead body typically comprises one or more insulated,conductive wire surrounded by an insulating outer sleeve. Eachconductive wire couples a proximal lead connector element with a distalstimulation and/or sensing electrode. An endocardial cardiac lead havinga single stimulation and/or sensing electrode at the distal lead end anda single conductive wire is referred to as a unipolar lead. Anendocardial cardiac lead having two or more stimulation and/or sensingelectrodes at the distal lead end and two or more conductive wires isreferred to as a bipolar lead or a multi-polar lead, respectively.

[0004] In order to implant an endocardial lead within a heart chamber, atransvenous approach is utilized wherein the lead is inserted into andpassed through a pathway comprising the subclavian, jugular, or cephalicvein and through the superior vena cava into the right atrium orventricle. It is necessary to accurately position the sense and/orstimulation electrode surface against the endocardium or within themyocardium at the desired site in order to achieve reliable sensing ofthe cardiac electrogram and/or to apply stimulation that effectivelypaces or cardioverts the heart chamber. The desired heart sites includethe right atrium, typically the right atrial appendage, the rightventricle, typically the ventricular apex, and the coronary sinus andgreat vein.

[0005] The transvenous pathway can include a number of twists and turns,and the lead body can be forced against bony structures of the body thatapply stress to it. Moreover, the heart beats approximately 100,000times per day or over 30 million times a year, and each beat stresses atleast the distal portion of the lead body. The lead conductors andinsulation are subjected to cumulative mechanical stresses, as well asmaterial reactions as described below, that can result in degradation ofthe insulation or fractures of the lead conductors with untoward effectson device performance and patient well being.

[0006] Early implantable, endocardial and epicardial, bipolar cardiacpacing leads employed separate coiled wire conductors in a side by sideconfiguration within a silicone rubber sheath and incorporated a lumenfor receiving a stiffening stylet inside the lumen of at least one ofthe conductor coils to facilitate advancement through the transvenouspathway. The stiffening stylet was advanced through a proximal connectorpin opening to stiffen the lead body during the transvenous introductionand location of the distal electrodes deeply inserted into the rightventricular apex and was then withdrawn. The relatively large diameterand stiff lead body provided column strength that was relied upon tomaintain the distal electrodes embedded into the trabeculae of the rightventricular apex. Fibrous tissue growth about the distal lead body wasalso relied upon to hold the distal pace/sense electrodes in position.

[0007] Similar atrial, J-shaped lead bodies were developed that reliedupon the lead body stiffness and shape to lodge and maintain distalpace/sense electrodes lodged into the right atrial appendage after thestiffening stylet was removed from the lead conductor lumen. In the caseof early J-shaped atrial leads formed of silicone rubber, the lead bodywas reinforced with an outward extending silicone rubber rib to maintainthe J-shape bend when the stylet was removed. In later J-shaped atrialleads, internally encased metal coils or wires have been employed tomaintain the J-shape bend.

[0008] Such relatively large and stiff lead bodies were disadvantageousin a number of respects. The available bio-compatible conductor materialalloy presented an impedance that limited current carrying capacity. Thelarge diameter body made it difficult to implant more than one leadthrough the venous system. The relatively high column strength was oftenstill insufficient to maintain the pace/sense electrodes in the atrialappendage or ventricular apex, and physicians often resorted to leavingthe stylets in place, resulting in fracture of the lead conductor andlead body sheath when the stylet wire broke. Once the lead bodiesfibrosed in, they were difficult to retract from the heart if theyneeded to be replaced. Finally, the lead conductors tended to fractureat stress sites, in bipolar leads sometimes due to stresses appliedunevenly to the side-by-side arrangement of the conductor coils.

[0009] In the efforts to solve these problems, more flexible lead bodieswere developed using smaller diameter coiled wire conductors and otherinsulating materials, most notably polyurethane compositions. Passiveand active fixation mechanisms incorporated were into the distal end ofthe endocardial lead to fix the electrode at a desired site in a heartchamber during the acute postoperative phase before fibrous tissuegrowth envelops the lead body. Passive fixation mechanisms, e.g., aplurality of soft, pliant tines that bear against the trabeculae in theright ventricle or the atrial appendage to urge the distal tip electrodeagainst the endocardium, do not invade the myocardium. Active fixationmechanisms are designed to penetrate the endocardial surface and lodgein the myocardium without perforating through the epicardium or into anadjoining chamber. The most widely used active fixation mechanismemploys a sharpened helix, which typically also constitutes the distaltip electrode, that is adapted to be rotated by some means from theproximal end of the lead outside the body in order to screw the helixinto the myocardium and permanently fix the electrode at the desiredatrial or ventricular site.

[0010] The side by side, bipolar, coiled wire lead body design was alsoreplaced by a coaxial configuration which is more resistant to fractureand smaller in diameter and which was typically formed of polyurethaneor silicone rubber inner and outer sheathes. More recently, each suchcoiled wire conductor of both unipolar and bipolar leads was formed of aplurality of multi-filar, parallel-wound, coiled wire conductorselectrically connected in common in an electrically redundant fashion asshown in commonly assigned U.S. Pat. No. 5,007,435, for example,incorporated herein by reference. Such redundant coiled wire conductorsof bipolar and multi-polar lead bodies are coaxially arranged about thestiffening stylet receiving lumen and insulated from one another bycoaxially arranged insulating sheaths separating each coiled wireconductor from the adjacent coiled wire conductor(s).

[0011] In the implantation of a cardiac device of the types listedabove, and in the replacement of previously implanted cardiac leads, twoor more transvenous cardiac leads are typically introduced through thevenous system into the right chambers or coronary sinus of the heart. Ithas long been desired to minimize the diameter of the transvenouscardiac lead body to facilitate the introduction of several cardiacleads by the same transvenous approach. Moreover, a number ofmulti-polar, endocardial cardiac leads have been designed to accommodatemore than two electrodes or to make electrical connection with othercomponents, e.g., blood pressure sensors, temperature sensors, pHsensors, or the like, in the distal portion of the lead. In addition,endocardial cardioversion/defibrillation leads were developed forunipolar or bipolar pacing and sensing functions and for deliveringcardioversion/defibrillation shocks to a heart chamber intended to beimplanted in a heart chamber or a cardiac blood vessel, e.g., thecoronary sinus. The increased number of separate polarity and insulatedcoiled wire conductors is difficult to accommodate in the conventionalcoaxial coiled wire conductor winding arrangement having a desired,small, lead body outer diameter. One approach involved the use ofseparately insulated, coiled wire conductors that are parallel-woundwith a common diameter and are separately coupled between a proximalconnector element and to a distal electrode or terminal as disclosed incommonly assigned U.S. Pat. No. 5,796,044, incorporated herein byreference.

[0012] Moreover, the use of thin polyurethane inner and outer sheathesalong with certain lead conductor alloys became problematic as thebio-stability of such lead materials in chronic implantation came intoquestion as described in commonly assigned U.S. Pat. No. 5,419,921. Ingeneral, it is acknowledged that there are a number of mechanisms fordegradation of elastomeric polyurethane pacing leads in vivo. One isenvironmental stress cracking (ESC), the generation of crazes or cracksin the polyurethane elastomer produced by the combined interaction of amedium capable of acting on the elastomer and a stress level above aspecific threshold. Another is metal ion induced oxidation (MIO) inwhich polyether urethane elastomers exhibit accelerated degradation frommetal ions such as cobalt ions, chromium ions, molybdenium ions and thelike which are used alone or in alloys in pacing lead conductors. Asexplained therein, certain polyurethane elastomers that have desirablecharacteristics for lead bodies are more susceptible to ESC and MIOdegradation than others that are less desirable. In the '921 patent, thepolyurethane elastomers that are susceptible to ESC and MIO degradationare coated or co-extruded with the less susceptible polyurethaneelastomers to form tubular sheaths having their inner and outer surfacesprotected by a less susceptible material layer.

[0013] All of the above considerations as to the increased complexity ofthe leads, the number of leads implanted in a common path, and desire toadvance leads deep in the relatively small diameter coronary veins haveled to efforts to at least not increase and optimally to decrease theoverall diameter of the cardiac lead body without sacrificingbio-stability, resistance to crushing forces, and usability. It has beenproposed to diminish the lead body further by eliminating the lumen forreceiving the stiffening stylet and by replacing the large diametercoiled wire conductors with highly conductive miniaturized coiled wireconductors, stranded filament wires, or cables formed of a plurality ofsuch stranded filament wires. In bipolar or multi-polar leads, each suchwire or cable extends through a separate lumen extending in parallelwithin a lead body sheath that maintains electrical isolation betweenthem.

[0014] Examples of such lead body insulating sheaths formed to enclose aplurality of straight, typically stranded, wire lead conductors,miniaturized coiled wire conductors or combinations of such straight andcoiled wire conductors are disclosed in U.S. Pat. Nos. 4,608,986,5,324,321, 5,545,203, and 5,584,873, all incorporated herein byreference. These patents and U.S. Pat. Nos. 4,640,983, 4,964,414,5,246,014, 5,483,022, and 5,760,341, all incorporated herein byreference, present a number of alternative designs of such strandedfilament wires or cables.

[0015] In the '873 patent, a unitary lead body insulating sheath isextruded having a plurality of spaced apart, outer lead conductor lumensthat extend longitudinally and in parallel to one another for receivingcoiled and/or straight wire conductors extending therethrough. Thesheath is extruded in a single piece of a single material, and a likeplurality of compression lumens that are preferably tear drop shaped areformed and extend longitudinally between the conductor lumens thatabsorb compression force that otherwise would crush a solid extrudedlead body sheath. In certain embodiments an inner, centrally disposedlumen is formed in the lead body sheath that can be employed as a leadconductor lumen or as a compression lumen that can be made large enoughin diameter to receive a stiffening stylet during introduction of thelead.

[0016] The above-referenced U.S. patent application Ser. No. 08/990,647discloses a lead body sheath formed of separate parts including anextruded strut member or core and a separately extruded tubular outertube. The core is extruded to form a plurality of longitudinallyextending grooves in which lead conductors may be located and theassembly of the core and lead conductors is fitted within the lumen ofthe outer tube which thereby encloses the core and holds the conductorsin the grooves. This construction simplifies the manufacture of the leadbodies, as it allows the conductors simply to be laid in the elongatedgrooves of the core rather than requiring that they be pushed or pulledalong the lengths of pre-formed lumens. In some embodiments, the core isprovided with a central, reinforcing strand, extending along the lengthof the lead body, providing for structural integrity and high tensilestrength. The core may be manufactured as a single extrusion, extendingthe entire length of the lead body, or may take the form of sequentiallyaligned multiple extrusions of differing materials to provide fordifferential stiffness along the length of the lead.

[0017] In all of these lead body designs, the adjacent conductor lumensare separated from one another by very thin webs of the extrudedinsulating material. Despite these improvements, the cumulative affectsof applied bending stresses can cause the extruded insulation webs ofthe lead body to split, thereby allowing the adjacent lead conductors tocontact one another and to short-circuit the electrodes they areconnected with. Each conductor lumen except for a centrally disposedconductor lumen is also separated from the outer surface of theinsulating body sheath by a thin web. This outer thin web can alsosplit, exposing the conductor to body fluids and tissues. The loss ofsupport of the lead conductors upon splitting of the lead body sheathwebs can also result in excessive bending and eventual fracture of theconductor. This problem is exacerbated when lead conductors of differingtypes each having a differing bending stiffness are enclosed in theouter lead conductor lumens leading to a lead body that is more flexiblewhen bent in one direction than when bent in another direction.

SUMMARY OF THE INVENTION

[0018] The present invention addresses these problems by forming a leadbody comprising an electrically insulating lead body sheath enclosingone or more lead conductors and separating the lead conductors fromcontacting one another. The lead body sheath is co-extruded in aco-extrusion process using bio-compatible, electrically insulating,materials of differing durometers in differing axial sections thereof,resulting in a unitary lead body sheath having differing stiffness axialsections including axial segments or webs or lumen encircling rings orother structures in its cross-section. The selection of the durometer ofthe materials and the configuration of the co-extruded lead body sheathsections may be used to control the geometric properties—e.g. bendingstiffness, torsional stiffness, axial tension-compression stiffness,shear stiffness, and transverse compression stiffness of the lead bodysheath. The lead body sheath is co-extruded to have an outer surfaceadapted to be exposed to the environment or to be enclosed within afurther outer sheath and to have a plurality of lead conductor lumensfor receiving and enclosing a like plurality or a fewer number of leadconductors.

[0019] In one embodiment, the lead body is co-extruded of a plurality ofsheath segments, each segment containing a lead conductor lumen andformed of a first durometer material, and of a web of a second durometermaterial extending between the adjoining boundaries of the sheathsegments. The web bonds with the adjacent segment boundaries to form theunitary lead body insulating sheath. The web may be formed byco-extrusion of a higher durometer material than the first durometermaterial.

[0020] In a further embodiment, the lead body sheath is co-extruded of aplurality of sheath segments, wherein each sheath segment contains alead conductor lumen and is formed of a selected durometer material,whereby the lead body sheath can be tailored to exhibit differingbending stiffness away from the lead body sheath axis in selected polardirections around the 360° circumference of the sheath body.

[0021] This embodiment is particularly suitable for use with leadconductors of differing types that have differing bending stiffnesses.In one application of this embodiment, the durometer of the sheathsegments are selected in relation to the lead conductor to compensatefor the lead conductor bending stiffness. For example, relatively stifflead conductors can be enclosed in segment lumens formed within segmentsof relatively low stiffness due to relatively low durometer materials,whereas relatively flexible lead conductors can be enclosed in segmentlumens formed within segments of relatively high stiffness due torelatively high durometer materials. In this way, the bending stiffnessof the lead body in all polar directions through 360° can be broughtinto equilibrium.

[0022] In a further application of this embodiment, it may be desired toform a lead body that does exhibit a bias to bend more readily in onepolar direction than in the other directions. In this case, one of thesheath segments can be co-extruded of a more flexible material than theother sheath segments, and can enclose a relatively flexible leadconductor within that sheath segment lumen.

[0023] In a third embodiment, the web of the first embodiment and thediffering stiffness sheath segment materials of the second embodimentcan be advantageously combined. In this way, additional control of crosssection geometric properties may be achieved by the use of differentdurometer materials not only for the sheath/webs, but also for eachsheath segment around the cross section of extruded lead body materialcontaining the lumens.

[0024] The sheath segments are preferably shaped as arcuate sections ofthe generally circular cross-section insulating sheath, and eachpreferably encloses a single lead conductor lumen. In a fourthembodiment, a centrally located lead conductor and/or stiffening styletreceiving lumen can also be formed of the same or differing durometermaterial as the sheath segments of the first or second embodiment orsurrounded by the web material of the first or third embodiment.

[0025] In these embodiments of the invention, the adjoining boundariesof the longitudinally extending sheath segments with one another or withan intervening web are in intimate physical contact and bonded with oneanother in the co-extrusion process.

[0026] Thus, the co-extruded lead body sheath may be constructed in avariety of ways and may or may not be enclosed in a co-extruded outersheath. The lead body sheath may contain a plurality of lumens eitherempty or containing electrical conductors, with or without a co-extrudedsheaths surrounding each lumen. The lead body sheath may be co-extrudedin sheath segments, sheath each segment connected by a radially orientedco-extruded sheath of various geometries which may or may not contactsheaths lining the lumens.

[0027] The co-extrusion of lead body sheath allows for selectivelcontrol of lead body stiffness for bending, torsion, axial tension orcompression, shear and transverse compression over the full length ofthe lead body or for a localized portion of the lead body length.Moreover, it allows the sheath segments and/or webs to act as barriersto prevent crack propagation in the lead body sheath which might lead toelectrical contact between the lead conductors producing a short orincorrect signals, or cracking to the outer surface allowing theelectrical lead conductors to be damaged by exposure to body fluids.

[0028] This summary of the invention and the advantages and featuresthereof have been presented here simply to point out some of the waysthat the invention overcomes difficulties presented in the prior art andto distinguish the invention from the prior art and is not intended tooperate in any manner as a limitation on the interpretation of claimsthat are presented initially in the patent application and that areultimately granted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] These and other advantages and features of the present inventionwill be more readily understood from the following detailed descriptionof the preferred embodiments thereof, when considered in conjunctionwith the drawings, in which like reference numerals indicate identicalstructures throughout the several views, and wherein:

[0030]FIG. 1 is a schematic illustration of a typical implantation of anIPG and endocardial lead system in which the lead conductor constructionand method of fracture detection of present invention is implemented;

[0031]FIG. 2 is a plan view of a typical endocardial pacing andcardioversion/defibrillation lead that incorporates the lead conductorsof the present invention; and

[0032] FIGS. 3-8 are cross-section views of the lead body of theexemplary lead of FIG. 2 illustrating various embodiments and variationsof embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0033] The present invention finds particular utility in the fabricationand implantation of cardiac leads, e.g., atrial and/or ventricularpacing leads and/or cardioversion/defibrillation leads having elongatedlead bodies and lead conductors that are subject to fracture. Preferredembodiments of such lead conductor fabrications of such endocardialcardiac leads that are implanted transvenously will be described indetail. But, it is to be understood that the present invention is notlimited to the same. The present invention can be implemented in thefabrication and use of other epicardial cardiac leads that are implantedsubcutaneously and in electrical leads intended to be disposed withinthe patients body, including nerve, brain, organ, and muscle stimulationleads.

[0034]FIG. 1 depicts a typical arrangement of a pacing or implantablecardioverter/defibrillator (ICD) system implanted in a patient 10, thesystem comprising a subcutaneously disposed implantable pulse generator(IPG) 12 and one or more endocardial atrial lead 14 and ventricular lead16. The IPG 12 is implanted in a subcutaneous location in the upperchest as shown in FIG. 1 or in the abdomen, and the proximal ends of theendocardial leads 14 and 16 are coupled with it. The distal end ofatrial lead 14 bearing one or more atrial pace/sense electrode is showndisposed generally in the atrial region of the patient's heart 18. Thedistal end of ventricular lead 16 bearing one or more pace/senseelectrode is disposed generally in the ventricular region of heart 18.The distal end of lead 14 can also be disposed in the coronary sinus andeven extend into a branching vein of the coronary sinus to dispose oneor more distal pace/sense electrode in relation to the atrium or theventricle to function as an atrial or ventricular pace/sense lead in amanner well known in the art. Alternatively, one or more of the leads 14and 16 can disposed epicardially about the heart 18. Moreover, one ormore of the endocardial leads 14 and 16 can include acardioversion/defibrillation electrode disposed at any of the abovedescribed locations.

[0035] An exemplary cardioversion/defibrillation lead 20 in which thepresent invention may be advantageously implemented and that can be usedin the locations of endocardial leads 14 and 16 is depicted in FIG. 2.Lead 20 is provided with an elongated insulating lead body 40,preferably fabricated of a plurality of co-extruded bio-compatibleelastomers, as described further below, and enclosing at least threelead conductors. Although not visible in FIG. 2, it should be noted thatthe elongated conductors passing through lead body 40 may be any of thevarious known available conductors for use in conjunction withimplantable electrical leads, including mono-filar or multi-filar coiledwire conductors, stranded wires formed of filaments, and the like asfurther described below with reference to FIG. 3.

[0036] An elongated cardioversion/defibrillation electrode 42, apace/sense ring electrode 44, and a pace/sense tip electrode 46 aresupported along a distal segment of the lead body 40 and are eachcoupled to a lead conductor located within the lead body 40. Electrodes42, 44 and 46 may correspond to any conventionally available pace/senseand cardioversion/defibrillation electrodes. When the lead 20 isintended for implantation in the right ventricular chamber, a fixationmechanism, e.g. the depicted soft, pliant tines 48 are provided to belodged within right ventricular trabeculae to maintain electrode 46 incontact with the endocardium of the right ventricle. Alternatively, anactive fixation mechanism, e.g., a retractable and rotatable helix, canbe substituted for the tines 48, and the distal tip electrode can befixed in the right atrial or ventricular heart chamber. The distalelectrodes of the lead 40 can also be advanced into a cardiac vessel,e.g., the coronary sinus, if no fixation mechanism is provided.

[0037] A connector assembly is formed at the proximal end of the leadbody 40 for making electrical and mechanical connection with the IPG 12of FIG. 1 in a manner well known in the art. The connector assemblycomprises a molded lead bifurcation which splits off two of theconductors within lead body 40 coupled to the distal pace/senseelectrodes 44 and 46 to a bipolar, in-line connector assembly 24 whichgenerally corresponds to the IS1 connector standard for pacing leads.Connector assembly 24 is provided with a first set of sealing rings 28,a connector ring 32, a second set of sealing rings 34, and connector pin36. Connector pin 36 is coupled to the lead conductor that extendsthrough the lead body 40 to the distal tip electrode 46. The connectorring 32 is coupled to the lead conductor that extends through the leadbody 40 to pace/sense ring electrode 44. The lead conductor coupled tocardioversion/defibrillation electrode 42 extends to connector assembly22 which comprises a set of sealing rings 26 and a connector pin 36. Theillustrated connector assemblies 22 and 24 are conventional elements andmay correspond to any of the numerous known electrical connectorassemblies provided on implantable medical leads.

[0038] In the specific context of the lead 20 illustrated in FIG. 2, thelead conductor coupling connector pin 32 to distal electrode 16preferably takes the form of a multi-filar, wire coil to allow passageof a stylet through a lumen of the wire coil. The lead conductorscoupling ring electrode 14 to connector ring 32 and coupling thecardioversion/defibrillation electrode 12 to connector pin 30 preferablytake the same form or the form of stranded cables formed of wirefilaments. But, the present invention is believed workable in thecontext of any of the numerous conductors known for use in implantableelectrical leads, in any combination with one another, with or withoutthe capability of receiving a stiffening stylet.

[0039] In conjunction with embodiments of the present invention whichemploy bundled, stranded conductors, interconnection of the conductorsto the electrodes and connector rings may be accomplished by crimping,swaging and/or welding, as known to the art. In particular,interconnection of bundled, stranded conductors to connectors andelectrodes may be accomplished according to U.S. patent application Ser.No. 08/439,332, filed May 11, 1995, by Swoyer et al., and in theabove-incorporated '014 patent and in U.S. Pat. No. 5,676,694, allincorporated herein by reference.

[0040] The lead body 40 of the present invention is realized in a numberof embodiments depicted in the cross-section views of FIGS. 3-8. In eachcase, the lead body 40 is formed as a lead body sheath that isco-extruded in a co-extrusion process using bio-compatible, electricallyinsulating, materials of differing durometers in differing axialsections thereof, resulting in a unitary lead body sheath havingdiffering stiffness sections or areas or structures in itscross-section. The lead body sheath is co-extruded to provide a leadbody outer surface 50 that is either exposed to the environment orenclosed within an outer sheath and to have a plurality of leadconductor lumens for receiving and enclosing a like plurality of leadconductors.

[0041] In FIG. 3, the lead body sheath 118 is co-extruded of three, forexample, sheath segments 106, 108 and 110, each segment containing alead conductor lumen and formed of a first durometer material. Sheath118 is co-extruded with a web 120 of a second durometer materialextending between the adjoining boundaries of the sheath segments 106,108 and 110 and bonding the adjacent segments together into the unitarylead body insulating sheath 118. The web 120 may be formed byco-extrusion of the second durometer material, typically a higherdurometer material than the first durometer material used to extrude thesheath segments 106, 108 and 110. The web 120 comprises three web arms122, 124 and 126 that adhere to the adjoining sheath segment boundaries,while separating the adjoining sheath segment boundaries from oneanother.

[0042] The lead conductors 112, 114 and 116 illustrated in leadconductor lumens 100, 102 and 104 in the cross-section view of FIG. 3are exemplary of lead conductor types that can be employed in thepractice of the invention. All of the lead conductors 112, 114 and 116are formed in an electrically redundant manner of a plurality of wirecoiled wires or stranded wire filaments that are coated on theirexterior surfaces with PTFE to facilitate inserting the lead conductorsthrough the lead conductor lumens 100, 102 and 104, respectively.

[0043] Two different versions of straight lead conductors 112 and 114are depicted in the lead conductor lumens 100 and 102, respectively.Each of the straight conductors 112 and 114 may take the form of abundled, stranded filament conductors disclosed in theabove-incorporated '986 and '321 patents or the '829 application, forexample. The invention may also be practiced using any of the numerousother stranded filament conductors known to the art. The strandedfilament conductors of the present invention can also be wound into aplurality of intertwined, parallel wound, coils that are electricallyconnected together as described above with respect to theabove-incorporated '983 and '022 patents, for example. The leadconductor 114 is formed of a straight, inner core filament 142surrounded by six outer filaments 144, 146, 148, 150, 152 and 154helically wound into a single 1×7 wire strand or cable. The leadconductor 112 is formed in the manner described in theabove-incorporated '414 patent of seven strands, and each strand isformed of seven filaments of smaller gauge than those forming leadconductor 114, resulting in a “7×7” wire cable of 49 total filaments.The straight, centrally disposed, core wire strand 128 is formed of sixouter filaments helically wound around a straight inner core filament.The six outer or perimeter wire strands 130, 132, 134, 136, 138, and 140are formed in the same manner as the core wire strand 128, that is, byan inner core filament surrounded by six outer filaments that arehelically wound about it. The six outer or perimeter wire strands 130,132, 134, 136, 138, and 140 are themselves wound helically around thestraight core wire strand 128.

[0044] Lead conductor 116 is formed in a parallel wound, multi-filar,coiled wire of the type disclosed in the above-incorporated '435 patent.The four parallel wound wires 156, 158, 160 and 162 are electricallyconnected together at the proximal connection with one of the proximalconnector elements and distal connection with one of the distalelectrodes.

[0045] Each of the conductive wires or filaments can be formed of asingle alloy material or formed as depicted with an inner core of ahighly conductive alloy or metal, e.g. silver, surrounded by an outersheath of another conductor, e.g., stainless steel or MP35N alloy, thatis more resistant to degradation using the DBS or the drawn-filled-tube(DFT) extrusion techniques described in the above-incorporated '044patent as is well known in the art. The current carrying capacity ofcardioversion/defibrillation lead conductors, e.g. the stranded filamentand cable conductors 114 and 112 formed in these ways is maximized forthe cross-section dimensions of the individual filaments and cables.

[0046] These differing lead conductors 112, 114 and 116 are merelyexemplary of different lead conductor types that may be employed in anycombination in the lead conductor lumens 100,102 and 104 and any furtherlead conductor lumens that can be formed in the insulating lead bodysheath 118. Such lead conductor types have differing outer diameters andbending stiffness characteristics.

[0047] The extrusion of the insulating lead body sheath 118 of the leadbody 40 can be effected in many ways in accordance with the presentinvention. In the first embodiment depicted in FIG. 3, the insulatingsheath 118 the lead body sheath is co-extruded of the plurality ofarcuate or pie piece shaped, sheath segments 106, 108 and 110 in whichthe lead conductor lumens 100, 102 and 104 are formed. Each segment 106,108 and 110 is formed of the same elastomeric material, e.g. apolyurethane a first durometer. The arms 122, 124 and 126 of web 120 areco-extruded of a second durometer material between the adjoiningboundaries of the sheath segments 106, 108 and 110 to bond them togetherinto the unitary lead body insulating sheath 118. The Y-shaped web 120is depicted schematically and can be of any suitable width thatstrengthens the lead body 40.

[0048] The web 120 is preferably formed by co-extrusion of a singlesecond durometer material, typically a higher durometer material thanthe first durometer material of the sheath segments 106, 108 and 110.However, it will be understood that the arms 122, 124 and 126 can beco-extruded separately of differing materials that are tailored, in thisinstance in durometer to complement and offset bending characteristicsof the lead conductors 112,114 and 116 or to otherwise affect thebending characteristics of the lead body 40.

[0049] Also, in the depicted view of this embodiment, the ends of thearms 122, 124 and 126 of web 120 extend to and becomes part of theexposed lead body surface 50. However, it will be understood that theouter ends of the arms 122, 124 and 126 can terminate within the sheathbody 118 such that the adjoining sheath segments 106, 108 and 110 mergetogether in a peripheral band adjacent to the exposed lead body surface50 and totally enclose the web 120.

[0050] In a first variation of the further embodiment depicted in FIG.4, the lead body sheath 218 is co-extruded of a plurality of sheathsegments 206, 208 and 210 enclosing lead conductor lumens 200, 202 and204, respectively. Each sheath segment 206 is formed of a selecteddurometer material, whereby the lead body sheath 218 as a whole can betailored to exhibit differing bending flexibilities away from the leadbody sheath axis (perpendicular to the plane of the cross-section view)in selected polar directions around the 360° circumference of the sheathbody 218. As a result of the co-extrusion process, the sheath segments206, 208 and 210 are bounded by and form the exposed surface 50 and theboundaries 220, 222 and 224.

[0051] The lead conductors 212, 214 and 216 are shown schematically inthis view and may take any of the known forms described herein orotherwise known in the art at the time of filing this application forpatent and that become known thereafter. This embodiment and thefollowing described variations are particularly suitable for use withabove described lead conductors of differing types that have differingbending stiffnesses. In one application of this embodiment, thedurometers of the sheath segments 206, 208 and 210 are selected inrelation to the lead conductor to compensate for the lead conductorbending stiffness. For example, relatively stiff lead conductors can beenclosed in segment lumens formed within segments of relatively lowstiffness due to relatively low durometer materials, whereas relativelyflexible lead conductors can be enclosed in segment lumens formed withinsegments of relatively high stiffness due to relatively high durometermaterials. In this way, the bending stiffness of the lead body 40 in allpolar directions through 360° can be brought into equilibrium.

[0052] In a further application of this embodiment, it may be desired toform a lead body that does exhibit a bias to bend more readily in onepolar direction than in the other directions. In this case, one of thesheath segments 206, 208 and 210 can be co-extruded of a more flexiblematerial than the other sheath segments, and can enclose a relativelyflexible lead conductor within that sheath segment lumen.

[0053] In the third embodiment of an insulating sheath 318 is depictedin FIG. 5, wherein a web 320 like web 120 of the first embodiment isemployed to strengthen the boundary between the adjacent lead conductorlumens 200, 202 and 204 while providing the differing stiffness sheathsegment materials of the second embodiment. The web 320 can be formed ofany suitable durometer material that is compatible with and can beco-extruded with the materials of the sheath segments 206, 208 and 210.

[0054] In the three variations of a fourth embodiment depicted in FIGS.6-8, a centrally located lead conductor and/or stiffening styletreceiving lumen 400 can also be formed of the same or differingdurometer material as the sheath segments of the first or secondembodiment or surrounded by the web material of the first or thirdembodiment. In FIG. 6, the insulating sheath 418 is formed in the mannerof the embodiment of FIG. 4 except that the centrally disposed lumen 400is formed therein. In FIG. 7, the insulating sheath 518 is formed in thesame manner as the embodiment of FIG. 6, except that the central lumen400 is surrounded by a tube 404 of differing durometer than those of thethree sheath segments 206, 208 and 210.

[0055] In the further variation of the insulating sheath 618 depicted inFIG. 8, a Y-shaped web 420 is formed of three arms 422, 424 and 426extending from the tube 404 surrounding the central lumen 400. The arms422, 424 and 426 are preferably extruded of the material of the tube404. However, they could be formed separately of materials tailored tocomplement the materials forming the sheath segments 206, 208 and 210.

[0056] In a further variation of FIG. 8, the sheath segments 206, 208and 210 could be formed of the same material as taught in the firstembodiment of FIG. 3. Moreover, the outer ends of the three arms 422,424 and 426 extending from the tube 404 surrounding the central lumen400 could be shortened so that the material used to extrude the segments206, 208 and 210 merge together in a band adjacent to the outer surface50.

[0057] In these embodiments of FIGS. 6-8, central lumen 400 would extendthrough lead body 40 to the lumen of proximal connector pin 36 and wouldbe adapted to receive a stiffening stylet or lead conductor or both asrepresented by element 402. For example, the coiled wire lead conductor116 of FIG. 3 would advantageously be disposed in central lumen 400 touse that lumen as a conductor lumen while still providing a lumen forreceiving a stiffening stylet in a manner well known in the art.

[0058] In the embodiments and variations depicted in FIGS. 4-8, thesheath segments 206, 208 and 210 are depicted as being formed ofdifferent durometer elastomeric materials that are co-extruded togetherwith or without a web 220 and/or tube 404 surrounding the central lumen400. It will be understood that two of the illustrated three segments206, 208 and 210 could be formed of the same durometer hardness material

[0059] It will also be understood that only two or more than three suchsheath segments can be formed in the lead body sheath in accordance withthe teachings of the present invention.

[0060] The elastomeric insulating sheathes 118, 218, 318, 418, 518, 618and the above-described variations thereof can be fabricated usingco-extrusion techniques that are well known in the art. For example,U.S. Pat. Nos. 4,790,831, 5,546,674 and 5,622,665 disclose exemplaryextrusion and co-extrusion techniques that are employed in theco-extrusion of side walls of catheter bodies for selectively alteringthe side wall characteristics around its circumference.

[0061] Acceptable polyurethane elastomers comprise polyether urethaneelastomers having a durometer on the Shore A durometer scale of at leastabout 80A or a substantially ether-free polyurethane elastomer. Theelastomer must also be hydrolically stable, not oxidize, and have atoughness in the range of polyurethanes generally. A suitable urethaneis Pellethane 2363-55D or Pellethane 2363-55DE of Dow Chemical Co. ofMidland, Mich. Polyurethanes essentially equivalent to Pellethane2363-55D are available from other sources such as B. F. Goodrich, Inc.The Pellethane 2363 family of polymers, including 2363-80A and 2363-55D,are composed of methylene bis-isocyanato benzene (MDI), butane diol (BD)hard segments and polytetramethylene ether oxide (PTMO) soft segments.The proportion of hard to soft segments is higher for the harder (Shore55D) polymer than for the softer (Shore 80A) material thereby providingfewer ether linkages which may be subject to in vivo degradation.

[0062] Preferably, the urethane is a substantially ether-freepolyurethane since stress cracking appears to have a relation to theether content of the polymer, with fewer ether linkages being desirable.A polymer without ether linkages may be made by substituting aliphatic,polycarbonate or polydimethylsiloxane groups for the polyether groups ofthe soft segments. Ether-free polyurethanes said to be suitable for invivo use are disclosed in-U.S. Pat. Nos. 4,873,308, 5,109,077, and5,133,742, and in published International Patent Application WO92/04390, all incorporated herein by reference in their entirety.Biostable ether-free polymers include PolyMedica's Chronoflex AL-80A andChronoflex AL-55D, the family of bistable polyurethanes disclosed in theabove-incorporated '308 patent and AKZO/ENKA'S PUR series ofpolyurethanes. These materials can be coated over the preferred leadinsulator material, Pellethane 2363-80A, by methods such as solutioncoating or co-extrusion.

[0063] In accordance with the method of the present invention, the leadbody sheath 118, 218, 318, 418, 518, 618 is fabricated to form each ofthe sheath cross-sections depicted in FIGS. 3-8 and above-describedvariations thereof using such co-extrusion techniques. The selected leadconductors are inserted through the sheath lumens for the length of thelead body sheath to form the lead body 40. Surface electrodes, e.g. thedistal ring electrode 44 and the elongated cardioversion/defibrillationelectrode 42 are formed over the distal exterior surface 50 andelectrically attached to the distal ends of the appropriate leadconductors. The distal tip electrode 46 is electrically attached to thedistal end of the appropriate lead conductor, and the distal electrode46 and fixation mechanism, e.g. the tines 48, are mechanically attachedto the distal end of the lead body 40. The proximal end of the lead body40 is attached to the proximal connector assemblies 22 and 24 in amanner well known in the art.

[0064] The principles of the present invention can also be applied tothe fabrication of an insulating sheath comprising an inner coresurrounded by an external sheath as disclosed in the above-incorporatedU.S. patent application Ser. No. 08/990,647. The inner core can befabricated in the co-extrusion process to have a plurality of sheathsegments as described above in reference to FIGS. 3-8 but having a leadconductor receiving groove, rather than a fully enclosed lead conductorlumen formed therein. The inner core and lead conductors fitted into thegrooves are encased within an outer tubing member of an elastomericmaterial.

[0065] Although particular embodiments of the invention have beendescribed herein in some detail, this has been done for the purpose ofproviding a written description of the invention in an enabling mannerand to form a basis for establishing equivalents to structure and methodsteps not specifically described or listed. It is contemplated by theinventors that the scope of the limitations of the following claimsencompasses the described embodiments and equivalents thereto now knownand coming into existence during the term of the patent. Thus, it isexpected that various changes, alterations, or modifications may be madeto the invention as described herein without departing from the spiritand scope of the invention as defined by the appended claims.

1. In a medical electrical lead for implantation within the living bodyof the type comprising an elongated lead body enclosing a plurality oflead conductors each extending between a distal a distal electrode orsensor element and a proximal connector element, the improvement in thelead body comprising: an elongated lead body sheath having an outersheath surface that is formed of a plurality of axial sheath segmentseach co-extruded of a bio-compatible, electrically insulating, material,the plurality of axial sheath segments extending in side by siderelation through the length of the lead body and bonded together atadjoining segment boundaries; a like plurality of elongated leadconductor lumens formed in and extending the length of each lead bodysheath segment to be enclosed thereby; and a like plurality ofelectrical lead conductors, each lead conductor extending through a leadconductor lumen.
 2. The medical electrical lead of claim 1 , wherein atleast two of the axial sheath segments are formed of materials ofdiffering durometers.
 3. The medical electrical lead of claim 2 ,wherein at least two of the lead conductors have differing bendingstiffnesses.
 4. The medical electrical lead of claim 1 , wherein saidplurality of lead conductors comprises a first lead conductor having afirst bending stiffness that differs from the stiffnesses of theremaining lead conductors of the plurality of lead conductors; and saidplurality of axial sheath segments comprises a first axial sheathsegment having a first leaden receiving the first lead conductor, thefirst axial segment formed of a material having a hardness that iscorrelated to the first bending stiffness.
 5. The medical electricallead of claim 1 , wherein each one of said plurality of lead conductorshave a bending stiffness that differs from the bending stiffnesses ofthe remaining lead conductors of the plurality of lead conductors; andeach one of said plurality of axial sheath segments are formed of amaterial having a hardness that is correlated to the bending stiffnessof the lead conductor received in the lead conductor lumen of the axialsheath segment.
 6. The medical electrical lead of claim 1 , furthercomprising a further lumen formed centrally in said lead body sheath. 7.The medical electrical lead of claim 6 , wherein at least two of theaxial sheath segments are formed of materials of differing durometers.8. The medical electrical lead of claim 7 , wherein at least two of thelead conductors have differing bending stiffnesses.
 9. The medicalelectrical lead of claim 6 , wherein said plurality of lead conductorscomprises a first lead conductor having a first bending stiffness thatdiffers from the stiffnesses of the remaining lead conductors of theplurality of lead conductors; and said plurality of axial sheathsegments comprises a first axial sheath segment having a first leadlumen receiving the first lead conductor, the first axial segment formedof a material having a hardness that is correlated to the first bendingstiffness.
 10. The medical electrical lead of claim 6 , wherein each oneof said plurality of lead conductors have a bending stiffness thatdiffers from the bending stiffnesses of the remaining lead conductors ofthe plurality of lead conductors; and each one of said plurality ofaxial sheath segments are formed of a material having a hardness that iscorrelated to the bending stiffness of the lead conductor received inthe lead conductor lumen of the axial sheath segment.
 11. The medicalelectrical lead of claim 1 , further comprising a tubular memberenclosing a further lumen formed centrally in said lead body sheath. 12.The medical electrical lead of claim 11 , wherein at least two of theaxial sheath segments are formed of materials of differing durometers.13. The medical electrical lead of claim 12 , wherein at least two ofthe lead conductors have differing bending stiffnesses.
 14. The medicalelectrical lead of claim 11 , wherein said plurality of lead conductorscomprises a first lead conductor having a first bending stiffness thatdiffers from the stiffnesses of the remaining lead conductors of theplurality of lead conductors; and said plurality of axial sheathsegments comprises a first axial sheath segment having a first leadlumen receiving the first lead conductor, the first axial segment formedof a material having a hardness that is correlated to the first bendingstiffness.
 15. The medical electrical lead of claim 11 , wherein eachone of said plurality of lead conductors have a bending stiffness thatdiffers from the bending stiffnesses of the remaining lead conductorsof-the plurality of lead conductors; and each one of said plurality ofaxial sheath segments are formed of a material having a hardness that iscorrelated to the bending stiffness of the lead conductor received inthe lead conductor lumen of the axial sheath segment.
 16. In a medicalelectrical lead for implantation within the living body of the typecomprising an elongated lead body enclosing a plurality of leadconductors each extending between a distal a distal electrode or sensorelement and a proximal connector element, the improvement in the leadbody comprising: an elongated lead body sheath having an outer sheathsurface that is formed of a plurality of axial sheath segments eachco-extruded of a biocompatible, electrically insulating, material, theplurality of axial sheath segments extending in side by side relationthrough the length of the lead body; and a web co-extruded of a furtherbiocompatible, electrically insulating, material extending between andbonded with adjoining boundaries of each axial sheath segment; a likeplurality of elongated lead conductor lumens formed in and extending thelength of each lead body sheath segment to be enclosed thereby; and alike plurality of electrical lead conductors, each lead conductorextending through a lead conductor lumen.
 17. The medical electricallead of claim 16 , wherein at least two of the axial sheath segments areformed of materials of differing durometers.
 18. The medical electricallead of claim 17 , wherein at least two of the lead conductors havediffering bending stiffnesses.
 19. The medical electrical lead of claim16 , wherein said plurality of lead conductors comprises a first leadconductor having a first bending stiffness that differs from thestiffnesses of the remaining lead conductors of the plurality of leadconductors; and said plurality of axial sheath segments comprises afirst axial sheath segment having a first lead lumen receiving the firstlead conductor, the first axial segment formed of a material having ahardness that is correlated to the first bending stiffness.
 20. Themedical electrical lead of claim 16 , wherein each one of said pluralityof lead conductors have a bending stiffness that differs from thebending stiffnesses of the remaining lead conductors of the plurality oflead conductors; and each one of said plurality of axial sheath segmentsare formed of a material having a hardness that is correlated to thebending stiffness of the lead conductor received in the lead conductorlumen of the axial sheath segment.
 21. The medical electrical lead ofclaim 16 , further comprising a further lumen formed centrally in saidlead body sheath.
 22. The medical electrical lead of claim 21 , whereinat least two of the axial sheath segments are formed of materials ofdiffering durometers.
 23. The medical electrical lead of claim 22 ,wherein at least two of the lead conductors have differing bendingstiffnesses.
 24. The medical electrical lead of claim 21 , wherein saidplurality of lead conductors comprises a first lead conductor having afirst bending stiffness that differs from the stiffnesses of theremaining lead conductors of the plurality of lead conductors; and saidplurality of axial sheath segments comprises a first axial sheathsegment having a first lead lumen receiving the first lead conductor,the first axial segment formed of a material having a hardness that iscorrelated to the first bending stiffness.
 25. The medical electricallead of claim 21 , wherein each one of said plurality of lead conductorshave a bending stiffness that differs from the bending stiffnesses ofthe remaining lead conductors of the plurality of lead conductors; andeach one of said plurality of axial sheath segments are formed of amaterial having a hardness that is correlated to the bending stiffnessof the lead conductor received in the lead conductor lumen of the axialsheath segment.
 26. The medical electrical lead of claim 16 , furthercomprising a tubular member coupled to said web and enclosing a furtherlumen formed centrally in said lead body sheath.
 27. The medicalelectrical lead of claim 26 , wherein at least two of the axial sheathsegments are formed of materials of differing durometers.
 28. Themedical electrical lead of claim 27 , wherein at least two of the leadconductors have differing bending stiffnesses.
 29. The medicalelectrical lead of claim 26 , wherein said plurality of lead conductorscomprises a first lead conductor having a first bending stiffness thatdiffers from the stiffnesses of the remaining lead conductors of theplurality of lead conductors; and said plurality of axial sheathsegments comprises a first axial sheath segment having a first leadlumen receiving the first lead conductor, the first axial segment formedof a material having a hardness that is correlated to the first bendingstiffness.
 30. The medical electrical lead of claim 26 , wherein eachone of said plurality of lead conductors have a bending stiffness thatdiffers from the bending stiffnesses of the remaining lead conductors ofthe plurality of lead conductors; and each one of said plurality ofaxial sheath segments are formed of a material having a hardness that iscorrelated to the bending stiffness of the lead conductor received inthe lead conductor lumen of the axial sheath segment.
 31. A method ofmanufacturing the lead body of a medical electrical lead forimplantation within the living body of the type comprising an elongatedlead body enclosing a plurality of lead conductors each extendingbetween a distal a distal electrode or sensor element and a proximalconnector element, the method comprising the steps of: co-extruding aplurality of axial sheath segments of a bio-compatible, electricallyinsulating, material each with a lead conductor lumen into an elongatedlead body sheath that is formed of the plurality of axial sheathsegments extending in side by side relation through the length of thelead body and bonded together at adjoining boundaries and enclosing alike plurality of elongated lead conductor lumens; and fitting each oneof a like plurality of electrical lead conductors through a leadconductor lumen.
 32. The method of claim 31 , wherein the co-extrudingstep further comprises the step of co-extruding at least two of theaxial sheath segments of materials of differing hardness.
 33. The methodof claim 32 , wherein at least two of the lead conductors have differingbending stiffnesses.
 34. The method of claim 31 , wherein said pluralityof lead conductors comprises a first lead conductor having a firstbending stiffness that differs from the stiffnesses of the remaininglead conductors of the plurality of lead conductors; and wherein theco-extruding step further comprises the step of co-extruding saidplurality of axial sheath segments with a first axial sheath segmenthaving a first lead lumen receiving the first lead conductor, the firstaxial segment formed of a material having a hardness that is correlatedto the first bending stiffness.
 35. The method of claim 31 , whereineach one of said plurality of lead conductors have a bending stiffnessthat differs from the bending stiffnesses of the remaining leadconductors of the plurality of lead conductors; and wherein theco-extruding step further comprises the step of co-extruding each one ofsaid plurality of axial sheath segments of a material having a hardnessthat is correlated to the bending stiffness of the lead conductorreceived in the lead conductor lumen of the axial sheath segment. 36.The method of claim 31 , wherein the co-extruding step further comprisesthe step of co-extruding a further lumen centrally in said lead bodysheath.
 37. A method of manufacturing the lead body of a medicalelectrical lead for implantation within the living body of the typecomprising an elongated lead body enclosing a plurality of leadconductors each extending between a distal a distal electrode or sensorelement and a proximal connector element, the method comprising thesteps of: co-extruding a plurality of axial sheath segments of abio-compatible, electrically insulating, material each with a leadconductor lumen together with a web of a further bio-compatible,electrically insulating, material extending between adjoining boundariesof each axial sheath segment into an elongated lead body sheath that isformed of the plurality of axial sheath segments extending in side byside relation through the length of the lead body and bonded together atadjoining boundaries by the web and enclosing a like plurality ofelongated lead conductor lumens; and fitting each one of a likeplurality of electrical lead conductors through a lead conductor lumen.38. The method of claim 37 , wherein the co-extruding step furthercomprises the step of co-extruding at least two of the axial sheathsegments of materials of differing hardness.
 39. The method of claim 38, wherein at least two of the lead conductors have differing bendingstiffnesses.
 40. The method of claim 37 , wherein said plurality of leadconductors comprises a first lead conductor having a first bendingstiffness that differs from the stiffnesses of the remaining leadconductors of the plurality of lead conductors; and wherein theco-extruding step further comprises the step of co-extruding saidplurality of axial sheath segments with a first axial sheath segmenthaving a first lead lumen receiving the first lead conductor, the firstaxial segment formed of a material having a hardness that is correlatedto the first bending stiffness.
 41. The method of claim 37 , whereineach one of said plurality of lead conductors have a bending stiffnessthat differs from the bending stiffnesses of the remaining leadconductors of the plurality of lead conductors; and wherein theco-extruding step further comprises the step of co-extruding each one ofsaid plurality of axial sheath segments of a material having a hardnessthat is correlated to the bending stiffness of the lead conductorreceived in the lead conductor lumen of the axial sheath segment. 42.The method of claim 37 , wherein the co-extruding step further comprisesthe step of co-extruding a further lumen centrally in said lead bodysheath.